Originally uploaded by Mic2006.
The classic cave as imagined by most generally forms in limestone. Limestone has a tight, interlocking structure that is impervious to water. Where the surface rock is limestone a karst environment usually develops. In this type of landscape the drainage is invariably beneath the surface where the surface flow penetrates to the underground along vertical crevices known as joints.
Within the rock the water travels horizontally along weaknesses known as bedding planes. The shape of an underground tunnel is usually a function of its movement in either of those two fissures and also other factors such as the gradient of flow and its situation either above or below the water table.
What is unusual about this tunnel is that it is in dolostone. Large passages rarely form in dolostone as the water passes through this rock much as it would a sponge. The rock is porus and the water is rarely concentrated in fissures. Solution of the rock and tunnel development is most likely where the water is pooled or running in crevices.
As you can see by the position of my feet, it was not a picture that was easily taken. I had to prop my head on the floor in the stream, press my body against the roof and photograph upside down.